How to test the glow plug relay
 

I guess I need to check the glow plug relay in my 1978 300CD. I have a volt/ohm meter, but I don't know that much about using them. Any help is appreciated.

It's not super hard. It's a w123 so I think it's the same as my old 300d but I could be wrong. Anyways, probably the same concept.

The goal of this part (the GP Relay) is to be the brain for your glow plug circuit. They need to be on for 20-30 seconds before you start the engine and turn off after it's started (exception the after glow upgrade for the newer MB diesels). I believe if you don't turn the key to start, the GPs will remain on for 1 minute.

To do this it's got a 5 wire harness with somewhat large wire (maybe 12AWG) going to the GPs. You can either pop the top off of this harness to expose the pins (Look at the wire harness and you'll see it has a snap lid on the back side of it) and measure on one of the 5 pins while it's plugged in to the GP relay or measure at the GP terminal itself.

What you're looking for is 12V at the GP cycle (first minute I believe after turning the key). Measure with a voltmeter (multimeter set to measure at least 12V). Put the black on the battery ground and the red on the battery Red just to check and make sure you're measuring 12V. Then have someone turn the key to Glow and measure at the GP relay's 5 pins or the Glow plugs themselves. If you have loop (series) style glow plugs your circuit will be a little different. If so you should measure voltage across the two wires on the GP but it won't be 12V. Rather 12V divided by the number of GPs in series.

Now if you don't measure voltage at the GPs, don't blame the relay yet--- here's why:

1. The relay is designed to NOT provide the GPs with power if more than 2 are burned out. (measure ohms across the GP -with it out of circuit- and you should see .7ohms approx on the new style (not old series type-if it is series type you'll see still close to this and Open Circuit still equals dead GP). If it's an Open Circuit or very high resistance, the GP is dead... you can also just use jumper cables on the GP and see if you get a red glow if it's not the series style GPs)

2. The connections on the relay may not be tight. Check them.

3. The connections at the GPs may be shorted or not in contact. Check them.

4. The FUSE on the GP relay may be shot. It's a big 80A (I think) silver colored strip fuse (about 1 1/4" by 1/4") that has a screw on either side. Measure across this to make sure it's good. You should see 12V on either side of the fuse or you can measure OHMS across it and see close to '0' ohms.

5. The wires could be shot. But this I'd doubt. Again though you can measure across them with an ohm meter and should get close to ZERO ohms.

But that should help you.

Here's more of a description of how your GP relay works if you're board:

It has some simple logic inside that does the following:

-Is the key turned to Glow?
-Do I measure a resistance that says I've got all but at least 1 or 2 glow plugs still working? (my guess as to how it knows the GPs are still good is by resistance)
-Has the starter engaged (the purple wire on the 4 wire harness senses the starter's voltage signal when it's energized)
-If YES to the first TWO and NO to the THIRD question, send 12V to the glow plugs for 1 minute.

I've fixed GP relays with corroded Relay copper terminals. All you need is a screwdriver to take the relay apart and sandpaper to scuff the terminals clean.

Anyways, hope this helps.
Nick

Also, there's probably more on here if you dig for it.

Feel free to ask if anything is confusing as well.

Did you just pick up the CD? Not on Craigslist-Springfield was it? ;) I saw a 78 a few weeks back. Those are cool old cars. Any 123 is.

Actually, I think the issue is with the ignition switch, which I have already ordered. I have had this car for about 2 years. Just converted to WVO, and then the electrical went south on me. I have been having intermittent starting problems for several months.

What is the basis of your belief that the relay won't close unless some minimum number of glow plugs are "working?"

Why do you believe that the relay opens concurrent with starter engagement?

I've seen it in action. Unplug 3 of your glow plugs and turn the key and your GP light won't come on. But I may be wrong about the relay not energizing the GPs. That may be incorrect. I do know the dash light will not illuminate.

As far as the starter engagement shutting off the GPs, that is what the purple wire on the 4 prong harness going to the relay does. It is a signal voltage to the GP relay to shut off.

Do you have the older style loop type glow plugs (big and have zig-zag wires connecting them) or pencile type glow plugs?
My understanding is that if you hava the loop style Glow Plugs that they are wired in series and if one goes bad the rest will not work.
Is your Glow Plug Relay under the hood on the fender well or under the dash board behind the instrument cluster?
Here is a site that might help:
http://www.pauldrayton.com/uploadfiles/merc/Service/W123/Index/617N/15EngineElec.html

Seeing the light in action is not the same as seeing the relay in action. The operation of the light and the operation of the relay are two different concepts. You unplug three of your GP's and see if your relay doesn't still work, even when the light doesn't.

There are two "signals" transmitted by the purple wire. One that the key has been placed in the start position and one when the key is released from the start position. It's the latter that terminates the glow cycle. (Whether the starter actually "engages" is irrelevant.)

Note: These observations might not be relevant to the original question, depending on the type glow timer installed on the subject vehicle.

When I went to an auto parts site and looked up the glow plugs for his year and model it showed the older Loop style plugs or those big pencile type conversion Glow Plugs.

If his Glow Plug relay is a silver metal box behind the Instrument Cluster; I have yet to see a trouble shooting thread on how to trouble shoot that type of relay. (Maybe the members over in the vintage Mercedes part of the forum know more about this relah.)

If it has the under the hood fender wall one the trouble shooting will be similar to the newer pencile type relays with the exeption that checking the Glow Plugs will be different.
I hope he will tell us what type of plugs/relay he has.

I can't figure out preglow plug relay problem.
 

All my notes are in bold. I would sure appreciate any ideas you guys can come up with.

Try looking at this site :thumbsup:

http://dieselgiant.com/glowplugrepair.htm

Did that.
 

Great web sight and very helpful. Everything checks out fine except there is no voltage coming from the 5 GP pins. In other words I know it is relay related. The only problem is it's brand new.

I get voltage from the battery to the large gauge wire that is bolted to the relay. The fuse checks our fine, voltage on both sides. Still no voltage to the 5 pins.

What this boils down to is the 4 pin wiring harness doing what it is supposed to do.

The four pins according to my wiring diagram (from a manual I payed way to much for).

Terminal 50 Cranking input. I believe it is from the starter.
Terminal 15 is from the run start input.
Terminal 31 Looks like the ground wire.
Terminal LA I believe this is the pin that sends info back to Pre glow lamp indicator yellow light.

I get a full 12 Plus volts from terminal 15 while the key is in in the position just before you turn over the car.

Terminal 50 provides only 10.5 volts. Could this be the problem? Or is the starter pulling the voltage down?

The function of the terminal 50 input is to interrupt the glow cycle following engine start. If the cycle is never starting, the problem is not associated with terminal 50. Sounds like a timer/relay failure to me; sometimes parts are bad right out of the box.

You should be able to hear the relay click closed and then open after 30-40 seconds.

Sure glad I saved the old one. Even though one of the contact points is shot it still acts differently than the new one.

Example

The new relay only engages the contacts while the engine is cranking. Once the key is turned off the contacts disengage.

The contact points on the old one (well two of them) make contact as soon as the the key is turned to the glow position.

Al I have to do is hold down the contact points on the old one and voltage is passed to the 5 pins.

Thanks. My luck that I would get a bad relay right out of the box.